Flat panel displays (FPDs) have been developed and commercialized and large, heavy displays employing a CRT (cathode-ray tube) have been replaced with FPDs in recent years. Among the widely used FPDs are liquid crystal displays (LCDs) applied to various portable electronic devices, notebook-size personal computers and miniature television sets. Plasma display panels (PDPs), as well as LCDs, have been developed and commercialized.
An EL display is one type of FPD. Efforts for developing EL displays began a relatively long time ago. However, EL displays have problems in displaying full-color pictures, luminance and short product life and hence have not been widely used.
A thin film of an inorganic compound (referred to as “inorganic compound film”) has been used as a luminescent layer, namely, an EL layer, for an organic EL device such as an EL display. An EL device employing a thin film of an inorganic compound needs a high driving voltage, has a low luminous efficiency and can display pictures in low luminance. Organic compound thin films capable of being driven by a low driving voltage and having a high luminous efficiency have become used as luminescent layers for EL devices in recent years. The organic EL device employing an organic compound thin film (the organic electroluminescent device) has a short product life. R&D activities have been made to develop materials for forming long-life organic luminescent layers. Organic EL devices practically competitive with LCDs have been developed.
Generally, a vacuum evaporation process has been used for forming an EL layer consisting of an organic luminescent layer, a carrier transport layer, which is used when necessary, and a carrier injection layer. However, organic compounds that can be used for vacuum evaporation are limited to those of a low molecular weight. In the organic EL device having an organic layer of an organic compound having a low molecular weight, crystallization or cohesion proceeds in the organic layer with time. Consequently, the device deteriorates and the life of the device is shortened greatly. An organic EL device provided with a luminescent layer formed of an organic compound having a high molecular weight, namely, an organic EL material, has been proposed. An organic EL material prepared by dispersing a carrier transport organic compound having a low molecular weight and a luminescent compound in an inactive polymer binder, and a polymerizing method of incorporating vinyl radicals into a carrier transport organic compound having a low molecular weight for polymerization have been proposed.
The EL layer of an organic EL material is formed by a wet coating process. A spin-coating process is a commonly used wet coating process. The spin-coating process is inefficient in solution usage and is not suitable for coating a surface of a large area with a film. Generally, the thickness of dry organic films serving as EL layers is in the range of 10 to about 200 nm. Therefore, the wet coating process for forming an organic film needs to use a coating solution having a very low concentration and a low viscosity. Therefore, a wet organic film formed by the wet coating process is highly fluidic and an organic film having an irregular thickness is liable to be formed. Since the organic film is very thin, even a small thickness deviation can be a large thickness error and it is highly possible that the small thickness deviation causes irregular luminescence. Therefore, when an organic film is formed by the spin-coating process, even a trace of a drop of the coating solution can cause irregular luminescence. It is very difficult for the spin-coating process to form a uniform film on a substrate of a size not smaller than 150 mm×150 mm without assistance.
Known wet coating processes other than the spin coating process include a die coating process, a bar coating process, a blade coating process, a roll coating process and bead coating process. Use of the die coating process for forming a film is mentioned, for example, in Patent documents 1 and 2.
A coating apparatus capable of carrying out the die coating process forms an organic film on a substrate by jetting a coating solution through the exit of a die placed on the substrate while the die is sliding along a surface of the substrate in a predetermined direction.
This known coating apparatus can form an organic film having a uniform thickness by jetting the coating solution through the exit of the die at a constant jetting rate.
Another coating apparatus capable of carrying out the die coating process disclosed in, for example, Patent document 3, moves a substrate during a coating operation.
This known coating apparatus shown in FIG. 7 includes a glass plate feed unit 101, namely, a feed means for feeding glass plates 100, a precision table 102 provided with air jetting holes, namely, a substrate lifting means for lifting up the glass plate 100 by air pressure, a glass substrate pushing block 104 provided with a precision linear motor 103, namely, a substrate moving means for moving the glass plate 100 at a predetermined coating speed, a die head 106 operated by a head lifting cylinder actuator 105, namely, a coating means for coating the glass plate 100 with a coating solution, a coating solution supply pump 107 and a coating tank 108, namely, coating solution supply means for supplying the coating solution to the die head 106, and a glass plate delivery unit 109, namely, a substrate delivery means for delivering the coated glass plate 100.
Patent document 1: JP 2003-10755 A (FIG. 1)
Patent document 2: JP 2002-153795 A (FIG. 1)
Patent document 3: JP 2002-346463 A ([0013], FIG. 1)
The foregoing known coating apparatus is able to form an organic film having a uniform thickness if the die or the substrate is slid at a fixed sliding speed during the coating operation. In some cases, the die cannot be slid at a fixed sliding speed by the motor, a principal component of a sliding means. If the die or the substrate is not moved at a satisfactorily constant speed, i.e., if the die or the substrate is moved at an irregular speed, the surface of the substrate is coated irregularly with the coating solution and, consequently, an organic film having an irregular thickness is formed. Since the EL layer, in particular, is very thin, small thickness irregularities can be large thickness errors, and large thickness errors are capable of causing irregular luminescence. The die needs to slide at a high sliding speed to increase the throughput of the coating process. It is expected that increase in the sliding speed intensifies the adverse effect of irregular speed. It is expected that the wider the surface to be coated, the greater is the adverse effect of irregular speed.